In acoustic well logging, it is customary to transmit an acoustic signal from a transmitter in a borehole containing fluid and to receive the resulting acoustic wave train at each of two or more acoustic receivers disposed in the borehole at locations spaced along the borehole axis from each other and from the transmitter. When the acoustic signal generated by the transmitter reaches the borehole wall, it produces an acoustic compressional wave and an acoustic shear wave, both of which, in general, will propagate through and be refracted in the formation traversed by the borehole until they are refracted back into the borehole fluid and are then detected at one or more of the receivers. In slow formations (in which the shear wave velocity is less than the acoustic velocity in the borehole fluid), the shear wave cannot refract back into the borehole, though, in general, the compressional wave can. When the acoustic signal generated by the transmitter reaches the borehole wall, it will also produce modal waves, as well as a Stoneley wave. These are surface waves strongest at the interface between the borehole fluid and the surrounding formation. The modal waves and the Stoneley wave will, in general, also propagate away from the transmitter and subsequently be detected at one or more of the receivers.
Any wave that is one of the mentioned types of waves detected at the receivers may be called an "arrival". The detected compressional waves in the borehole fluid caused by refraction of compressional waves (sometimes referred to as "P waves" hereinafter) in the formation traversed by the borehole will be referred to herein as "compressional wave arrivals". The detected compressional waves in the borehole caused by refraction of shear waves (sometimes referred to as "S waves" hereinafter) in the formation will be referred to herein as "shear wave arrivals". The detected compressional waves in the borehole fluid caused by Stoneley waves will be referred to herein as "Stoneley wave arrivals".
Thus, the full acoustic wave train received at each receiver will, in general, be a composite signal which includes a compressional wave arrival, a shear wave arrival, and a Stoneley wave arrival. Usually in such composite signal, the compressional wave arrival will be detected first and thus be the first arrival. The shear wave arrival will usually be the second arrival and the Stoneley wave arrival the last arrival.
FIG. 1 shows twelve full wave train signals of the type described above, each received at a different acoustic receiver spaced at a different distance from a common transmitter. Increasing distance downward on FIG. 1 represents increasing actual distance away from the transmitter along the borehole axis. Increasing horizontal distance toward the right of FIG. 1 represents increasing time following transmission of an acoustic pulse by the transmitter.
In a conventional processing scheme, the early portion of each full wave train signal of the type shown in FIG. 1 is analyzed to determine the level of acoustic and electronic noise present. The first arrival is identified as the earliest local maximum to exceed the noise level by some preselected amount. Two or more cycles of the first arrival are analyzed to determine its magnitude and the search is continued for later arrivals, using other preset trigger levels. Travel times and attenuation lengths are computed from the time and magnitude of the arrivals in adjacent receivers. Accuracy may be increased by, for example, calculating cross-correlation or semblance functions for small time windows about the picked arrivals.
The conventional processing method described above suffers from several drawbacks, including the following:
(1) The trigger levels which must be preset are complicated functions of the transmitter strength, receiver sensitivity, geometric falloff and formation attenuation. There is no reason to expect triggers set in one interval to remain valid in any adjacent interval. Cycle skipping is a serious problem unless the logging tool provides redundant measurements from several receivers. To make frequent changes in the trigger levels during processing is nearly as time-consuming as picking the arrivals by hand.
(2) In order to make the shear and Stoneley arrivals more distinct from the other modal energy, the data must be filtered in software. This extra work increases the computation time by a factor of about twenty.
(3) One must also preset time windows to evaluate the noise levels and to search for the various arrivals. Attempts to adjust these windows during the arrival picking process run the risk of confusing compressional and shear wave arrivals altogether. In this respect, the conventional processing method is little better than picking the arrivals by hand.
(4) The conventional method is slow. Even without software filtering, a mainframe computer can only perform the algorithm at about half the speed at which the data is collected.
(5) The conventional method assumes a very specific model of wave propagation in a borehole. There is no provision for dispersive modal energy, reflections from horizontal interfaces or refractions from the edge of the invaded zone. Conversely, the method does not automatically eliminate these arrivals.
(6) The conventional processing method discards nearly all of the data from a multiple receiver, full waveform tool. This last point requires some elaboration. Conventional full waveform acoustic logging systems produce large amounts of data. For example, such conventional systems may typically digitize received wavetrains having a length of 10 milliseconds, at 5 microsecond sample rate. Such a system may typically have twelve receivers, and may typically be used to log an interval 3,000 feet or more in length along a borehole, at half foot intervals. The amount of resulting data is several hundred thousand times the amount of data typically obtained by using a conventional induction or gamma ray tool to log the same interval. Rather than process all this data, the conventional method discards almost all of it. The log analyst is left with only a few millionths of the data and with no way to recover from faulty presets in the computer program.
The method of the present invention, in contrast, facilitates the processing and displaying of data from full acoustic wavetrain logs in a manner which is not only faster than the conventional technique described above, but is such that more of the useful information in the wavetrains to be processed is retained in the resulting display. The present invention also avoids the problems, arising as the result of performing the conventional technique, which are associated with the selection of incorrect peaks.